PROJECT SUMMARY The purpose of this core is to provide essential common facilities and expertise for the immunopathological studies in all 3 projects. The primary role is to document and characterize: 1) The status of the heart, lung, kidney and skin allografts (nature of infiltrate, presence of acute or chronic rejection, including a humoral component and the quantitation of lesions. 2) Systemic effects of the protocols (toxicity, complications, BK viral infection, PTLD) 3) Intragraft markers that predict acceptance vs. rejection 4) Mechanisms by which intragraft cells may promote rejection or acceptance The pathology studies in Core A are essential to evaluate the status of the allografts, particularly the in situ events in the heart, lung and kidney grafts. The studies are critical to determine the nature and location of the cells infiltrating the graft, the role of antibodies in rejection and systemic toxicity of treatment. The techniques that will be utilized include routine histology, immunohistochemistry, immunofluorescence, and electron microscopy. Immunoperoxidase techniques with a panel of mAbs will distinguish the infiltrating cell types (Teff, Treg, Breg, et al), adhesion and cytokine molecules and receptors, complement deposition (C4d), and endothelial activation markers (e.g., pERK). Analysis will be enhanced by whole slide digital scans and morphometry. The pathology results will be correlated with functional and molecular studies done in the projects. In addition, complete necropsies will be done on all animals, with samples from all major organs analyzed by routine and special techniques as indicated.